An extensive research and development has been conducted on the use of lithium metal, which is capable of realizing high voltage and high energy density, as the negative electrode for non-aqueous electrolyte secondary batteries. This has lead to commercialization of lithium ion batteries that use a graphite material, which is capable of reversibly absorbing and desorbing lithium and excellent in cycle life and safety, in the negative electrode.
However, the useful capacity of batteries using a graphite material-based negative electrode is approximately 350 mAh/g, which is very close to the theoretical capacity (372 mAh/g) of a graphite material. Thus, as long as a graphite material is used in the negative electrode, a dramatic increase in capacity is not feasible. Meanwhile, the functions of portable appliances are becoming increasingly sophisticated, and the required capacity of non-aqueous electrolyte secondary batteries serving as the energy source of such appliances tends to increase commensurately. Therefore, in order to achieve higher capacities, negative electrode materials having capacities higher than that of graphite are necessary.
As materials offering higher capacities, alloy materials containing silicon (Si) and alloy materials containing tin are currently receiving attention. These metal elements are capable of electrochemically absorbing and desorbing lithium ions and capable of charge/discharge with capacities that are significantly higher than that of a graphite material. For example, it is known that silicon has a theoretical discharge capacity of 4199 mAh/g, which is 11 times as high as that of graphite.
Therefore, research is now underway on batteries that use a silicon-containing negative electrode active material, together with conventional lithium secondary battery components such as a lithium cobaltate positive electrode and a non-aqueous electrolyte composed of a 1 mol/L lithium hexafluorophosphate and a mixed solution of ethylene carbonate and ethyl methyl carbonate. However, if such batteries are stored at high temperatures, particularly in a discharged state, they are highly susceptible to deterioration. Therefore, they have a problem of battery malfunction after storage.
In order to avoid this problem, it is preferable to minimize the discharge potential of the negative electrode. For example, Japanese Laid-Open Patent Publication No. Hei 11-233155 (Patent Document 1) proposes minimizing the capacity loss due to charge/discharge cycles by using SiO as the negative electrode active material and controlling the end-of-discharge potential of the negative electrode at 0.6 V or lower relative to a Li electrode.
However, if the discharge potential is limited as in Patent Document 1, since the average discharge potential of SiO is 0.4 V to 0.5 V relative to a Li electrode, only about a half of the capacity SiO inherently has is utilized, so that the inherent high capacity of SiO is sacrificed.